Digital cellular radio systems of the second generation, such as the known GSM (Global System for Mobile telecommunications), originally relied on all the base stations being substantially equal and all the terminals or mobile stations being substantially equal in functional capabilities At the priority date of this patent application there is a clear developmental trend towards greater versatility where both base stations and mobile stations may implement various selections of a multitude of functional features. Examples of such features include but are not limited to support for different radio access technologies (e.g. GSM 900, GSM 1800, GSM 1900, IMT2000; the numbers denote approximate frequency ranges in MHz and IMT comes from International Mobile Telecommunication) and support to different radio access features (e.g. ECPRS, ECSD, HSCSD, LCS; Enhanced General Radio Packet Service, Enhanced Circuit Switched Data, High-Speed Circuit Switched Data, Localized Cellular Services). It is assumed that the variety of functional features supported by base stations and mobile stations will even greatly expand in the future. In this patent application we use the general term “capability information” to denote all such information which a communication device in a cellular radio system may announce to other communication devices in order to make them aware of its ability to support and/or exploit certain functional features.
It has been regarded as necessary for the base stations and mobile stations to exchange a certain amount of capability information so that on one hand the network is able to offer to the mobile station such services which the mobile station is both capable and willing to utilize, and on the other hand the mobile station may optimize its operation for example by selecting a cell, from a list of potential candidate cells, which is most optimal in terms of low charges, adequate data rates or some other criterion.
In the following we consider the problem of providing the network (which is a general term describing the fixed parts of a cellular radio system) with capability information concerning the mobile stations operating within the coverage area of the network. From the known GPRS (General Radio Packet Service) specifications there is known a procedure where a mobile station composes an information element known as the RAC IE (Radio Access Capability Information Element) and includes it into certain messages transmitted to the network. Especially MM, RLC/MAC and BSSGP layer messages comprise the RAC IE, where MM, RLC/MAC and BSSGP are names of certain protocol layers and come from Mobility Management, Radio Link Control/Media Access Control and Base Station System GPRS Protocol. Similarly from the known GSM specifications there is known the procedure of sending, from the mobile station to the network, a CM IE or ClassMark Information Element within MM layer messages. The purpose of both the RAC IE and the CM IE is to announce the capabilities of the mobile station to the network.
The continuous expansion of the “feature space” from which an arbitrary selection of features may be supported by the mobile station means that in worst cases the RAC IE and/or the CM IE in their known form may easily become very large. Their inclusion into every MM, RLC/MAC and BSSGP layer message may begin to consume a remarkable portion of radio resources, which in general are scarce and should be reserved to the transmission of only absolutely necessary information. Additionally the introduction of new parts to the already specified information elements may necessitate tedious restandardization.
A straightforward partial solution to the above-mentioned problem would be to reduce the number of transmitted messages containing the RAC IEs and/or the CM IEs. This could be done for example by shortening the list of protocol layers the messages of which are allowed to contain said information elements, or by stating that only every Nth message of a certain protocol layer is allowed to contain said information elements, where N is a positive integer. However, such solutions would seriously cut down the possibilities of providing the network with updated capability information from the mobile stations, which in turn could cause degradation in the subjective level of service which a user experiences.
Another approach to the problem of informing a base station about the capability of a mobile station is to have the functional features of each mobile station listed in a database somewhere in the network, and signalling the capability record concerning a particular mobile station to a base station (or other interested network element) whenever a mobile station has performed a location update within the domain administrated by that base station or other network element. This approach has the drawback of causing a large amount of signalling within the network. It is also inflexible regarding such situations where the capabilities or preferences of a mobile station change, because the central database of capabilities should be updated after each change.